Developed by the Facility that created X-23, the trigger scent was designed to force X-23 into an involuntary primal rage to kill any targets who were laced with it.
X-23 was put through brutal conditioning that bordered on torture from at least age eight by Zander Rice, forced to respond to the trigger scent on a subconscious level. Once she detects the scent, she essentially blacks out as her feral killing instincts take over, killing anyone marked with the chemical and when the effects wear off, she returns to full consciousness with no memory of what happened.
The trigger scent was not used on all of her assassination missions, but on targets who were exceptional or those who X-23 couldn't be trusted to kill. Such targets included the mob assassin Fade, X-23's sensei, X-23's mother Sarah Kinney and Facility agent Desmond Alexander. Megan and Debbie Kinney were able to survive being laced with trigger scent by washing it off their bodies and clothing before X-23 was able to kill them.